City breaks

What a deal: just spend pounds 250 at Sainsbury's over the next seven weeks, and save a fortune on British Airways flights. Between next January and May, you and four friends can travel to Europe and North America for half price. To support the promotion, BA and Sainsbury's cite examples like London-Amsterdam for pounds 56 return, rather than pounds 112

To dispense with that claim first: one thing you certainly can't do is travel between London and Amsterdam for pounds 56. Though the newspaper ads fail specifically to mention it, there is the small matter of pounds 18.40 in tax, increasing the ticket price to Amsterdam by one-third.

British Airways says the in-store leaflet promoting the deal mentions tax five times.

Suppose you accept the airline's view that fares should be an exception to the general rule that consumer prices include tax. Surely a deal offering half-price travel is a bargain?

That depends on the price being halved. On any British Airways plane between London and Amsterdam, passengers pay a wide range of fares - from pounds 328 for an unrestricted business-class ticket, down to zero for Air Miles fliers, airline staff and freeloading journalists. Like any other business, BA tries to maximise its earnings. The airline does this by organising its seats into a large number of classes for each flight.

You and I may think there are just two - Club Europe, and "Euro Traveller", as economy is called - but within the airline's computer system each of these is subdivided according to the fare paid. The idea is to sell as many expensive seats as possible, then to offload the rest at progressively lower fares. On a Friday evening, for example, BA can sell most seats at high fares; conversely, Wednesday lunchtime departures are hard to fill at any price.

The supermarket tie-in aims at filling these empty seats. But don't imagine that you can just take the best BA fare and halve it. The leaflet, but not the ads, points out that World Offer fares are excluded - so forget the pounds 69 ticket between London and Amsterdam currently on offer. The next lowest fare for the dates when the Sainsbury's offer applies is pounds 83. But hang on - BA claims the normal fare is pounds 112.

"These fares are highly competitive," says a BA spokeswoman. "We're not marketing them as the cheapest fares, but as half-price fares. They are based on the lowest prices excluding World Offers and V-class, and this is clearly stated in our promotional literature."

What's V-class, then? "V-class is a year-round fare that carries restrictions and has limited availability."

So can travellers infer that the "half-price" fares are unrestricted, and offer unlimited availability? Sadly, no.

Some of the promotion's fares, like Gatwick-Athens for a maximum of pounds 128 including tax, are excellent - which is why I could be seen pedalling precariously back from Sainsbury's at Nine Elms in south London on Thursday with a year's supply of pasta. But I reckon BA is being unnecessarily economical about its economy fares.